This invention pertains to the art of heating cables and more particularly to self-regulating heating cables.
The invention is particularly applicable to heating cables comprised of conductive polymeric compositions, and will be described with particular reference thereto. It will be appreciated, however, that the invention has broader applications and may be advantageously employed in other environments.
A self-regulating heater is essentially comprised of a pair of parallel wires or conductors which are joined together by a semi-conductive, substantially polymeric material. The resistance of the material changes relative to changes in temperature. When a power source is connected to the heater and a particular voltage is applied, the heater begins to heat and thus changes its own internal resistance. The output of the heater at a given voltage thus changes in relation to the heat transfer between the heater and surroundings. The heater's construction allows it to limit itself at any given temperature below its continuous use temperature.
Self-regulating heating cables of the present invention are typically used in association with fluid carrying pipes, although they are also used to provide underground warmth for gardens and walkways. The purpose of the heating cables is to maintain a temperature which does not drop below a predetermined minimum.
In the case of water pipes, for example, it is important to prevent water within the pipe from freezing and either blocking the flow of water therethrough, or bursting the pipe to cause extensive damage. Self-regulating heating cables of the present invention are wrapped around the pipes, typically in a spiraled fashion, and serve to provide heat to the pipe as the surrounding temperatures decrease. Once surrounding temperatures begin to rise, a lesser amount of heat is transferred to the water pipe.
Heretofore, self-regulating heating cables have not dependably and predictably prevented fluids within pipes from freezing. That is to say, they have not been able to provide an even, predictable distribution of heat along their entire lengths. In some instances, portions of existing heating cable have not transferred any heat at all.
More recently, it has been discovered that predominantly uniform control of self-regulating heating cables can be obtained by substantially evenly distributing conductive filler throughout the polymeric coating which covers the parallel wire conductors prior to extrusion. A uniform dispersion of conductive material throughout a self-regulating heating cable permits the cable's resistance level to approach uniformity as well. With this knowledge in hand, previous attempts have been made to manufacture consistently operable self-regulating heating cables.
One such prior method involves compounding a given percentage of conductive carbon black with a polymer system, and extruding the resultant compound directly into cable. This method yields a product having non-uniform resistance. The non-uniformity results from the inherent difficulty of metering the amount of conductive filler dispersed in the polymer, as well as the extreme sensitivity of resistance due to mixing. Simply compounding the filler with the polymer does not result in uniform dispersion of filler. As little as a 0.25 percent deviation of conductive filler concentration throughout the system results in a 1.times.10.sup.2 ohms-feet magnitude resistance oscillation. Such an inconsistent variation in resistance within a single heating cable is undesirable and renders the resulting heating cable defective for many applications.
A second previously existing method which has been followed in an effort to obtain a desired resistance throughout a self-regulating heating cable calls for the use of standard color concentrating techniques. That is, a first polymeric constituent which includes a relatively high percentage of conductive filler is mixed with a second polymeric constituent which includes no conductive filler. The constituents are blended together. The resistance of the mixture can then be adjusted by adding or subtracting the constituent containing the conductive filler.
While this second method offers a greater dissemination of filler throughout the polymer, extreme resistance uniformity problems similar to those developed in the first method arise. Heaters produced using the polymeric compound blended in accordance with the second method are essentially undesirable.
A third method for improving the uniformity in a self-regulating heating cable involves the modification of the second method discussed above. In this third case, however, both constituents contain some conductive filler. That is, the conductivity of the first constituent is slightly greater than the desired resultant conductivity. Similarly, the conductivity of the second constituent is slightly lower than the desired resultant conductivity. Appropriate amounts of the two constituents are combined to obtain a polymeric material which will approximate the desired resultant conductivity throughout.
Because the probability of having the desired amount of conductive filler at any point in the extruder increases by using the third method, results more agreeable than those of the second method can be obtained.
This third method does, however, also have its shortcomings. Although the resistance uniformity offered by this method is much better than that of either the first or second methods mentioned above, the method does not provide sufficiently consistent desired resistance ranges to provide a feasible working product. As a result, scrap rates of unusable self-regulating heating cable are rather significant.
It would be desirable to develop a compound suitable for production of a self-regulating heating cable such that the compound would comprise a substantially uniform dispersion of conductive filler within a polymeric constituent.
It would be further desirable to develop a self-regulating heating cable offering a substantially uniform resistivity therethrough such that the cable would provide evenly dispersed heating.
The present invention contemplates a new and improved method which overcomes all of the above referred problems and others and provides a self-regulating heating cable comprised of a polymeric constituent which offers substantially uniform resistivity throughout.